phsl / new-api

Remove the 5-image upload restriction in playground and enhance UI consistency

Changes:
- Remove 5-image limit constraint from ImageUrlInput component
- Update hint text to remove "maximum 5 images" references
- Add custom scrollbar styling for image list to match site-wide design
- Apply consistent thin scrollbar (6px width) with Semi Design color variables
- Maintain hover effects and rounded corners for better UX

Breaking Changes: None

Files modified:
- web/src/components/playground/ImageUrlInput.js
- web/src/index.css

This change allows users to upload unlimited images in playground mode while
maintaining visual consistency across the application's scrollable elements.